1. Field of the Invention
The present invention relates to an electric driving unit capable of eliminating backlash generated in a gear train, and more particularly to an electric driving device adapted for precisely conducting automatic focusing or feeding operation in a camera, without the influence of backlash.
2. Related Background Art
In the field for example of automatic focusing devices for automatically focusing a photographing lens, there is already known a structure of rotating a helicoid ring with a small motor through a gear train. In such devices, since precision on the order of several microns is required in the lens position control, a mechanism for detecting the lens movement has to have a resolving power of several microns, measured in the amount of lens movement. Such a detecting mechanism is generally provided at the first gear of the gear train, and certain backlash is inevitable in the meshing of gears. Consequently, in a lens driving operation, even if the detected amount of lens movement is correct, the actual amount of lens movement may be decreased due to free plays of gears resulting from the backlash thereof. Also if the amount of lens movement& is as small as the magnitude of said play, the lens may not be moved at all even though the motor is driven.